html-doctype
Doctype
In HTML, the doctype is the required "<!DOCTYPE html>" preamble found at the top of all documents. Its sole purpose is to prevent a browser from switching into so-called "quirks mode" when rendering a document; that is, the "<!DOCTYPE html>" doctype ensures that the browser makes a best-effort attempt at following the relevant specifications, rather than using a different rendering mode that is incompatible with some specifications.
Definition and Usage
All HTML documents must start with a <!DOCTYPE>
declaration.
The declaration is not an HTML tag. It is an "information" to the browser about what document type to expect.
In HTML5, the <!DOCTYPE>
declaration is simple:
<!DOCTYPE html>
In older documents (HTML 4 or XHTML), the declaration is more complicated because the declaration must refer to a DTD (Document Type Definition).
Doctype Declaration
<!DOCTYPE html>
<html>
<head>
<title>Title of the document</title>
</head>
<body>
The content of the document......
</body>
</html>